With the promotion of the concepts of greenery and low-carbon economy being popular in the world, the operators requires more and more to lower the power consumption of the wireless communication system. In the wireless communication system, the radio frequency power amplifier (abbreviated as power amplifier) in the base station device is one of the core modules of the whole system, and an important parameter of the radio frequency power amplifier is power amplifier efficiency. Data analysis indicates that in the whole base station device, the power consumption of the power amplifier portion occupies about 60% of the whole power consumption, thus, improving the efficiency of the power amplifier is the most effective means to reduce the power consumption of the base station device and reduce the operating expense (OPEX) of the operators. Therefore, in face of the increasingly fierce market competition of the wireless communication, high efficient radio frequency power amplifier technology has become one of the competition focal points of the wireless communication industries.
Doherty power amplifier is a high efficient power amplifier technology most widely applied currently in the wireless communication system, and it is invented by an American electronic engineer William H. Doherty in 1936. However, during the following about thirty years, people has switched their attention. Until the end of 1960s, with the development of the communication technology, especially satellite communication, the efficiency and linearity problems of the power amplifier are proposed again in a new history occasion, and the Doherty power amplifier is unearthed again and widely applied in the communication and broadcast system in 1970s. Until today, the combined application of Doherty power amplifier and digital pre-distortion (DPD) technology has become the mainstream architecture form of the high efficient power amplifier of the base station in the wireless communication system.
The principle of the conventional Doherty power amplifier is as shown in FIG. 1, and it mainly includes a power drive circuit (D1 . . . Dn in FIG. 1), a power splitter (S in FIG. 1), a carrier amplifier (C in FIG. 1, also referred to as main power amplifier), a peak amplifier (P in FIG. 1, also referred to as auxiliary power amplifier), and a power combiner it (C′ in FIG. 1), etc. The basic concept of the conventional Doherty power amplifier is active load pull. The carrier amplifier works in class B or class AB, and the peak amplifier works in class C, and both of them undertake different input signal powers respectively and make the two power amplifiers work in their own saturation areas as far as possible, thus ensuring that the whole power amplifier maintains relatively high efficiency within a input signal power range which is as large as possible and at the same time ensuring a certain linearity.
Doherty power amplifier includes three working states: 1) small signal area, when the input signal is relatively small, the peak amplifier is in a cutoff state, the carrier amplifier works in class AB, and at this moment, the carrier amplifier works in the maximum efficiency match state; 2) load modulation area, when the input signal increases to a certain extent, the carrier amplifier gradually transits from the amplification area to the saturation area, the peak amplifier gradually transits from the cutoff area to the amplification area, the loads of these two amplifiers are instable, and the resistances of the loads change with the changes of the powers; and 3) saturation area, with the increasing of the input signal, the carrier amplifier and the peak amplifier finally work in the saturation state, both of them correspond to the load of 50Ω, and the output powers are added.
The conventional Doherty power amplifier has a relatively big disadvantage, i.e. due to the discretion of the input standing waves of the power amplifier device during the batch manufacture of the power amplifier, the consistency of the output of the power splitter (S in the figure) is affected, which will cause the mismatching when combining the output powers of the carrier amplification branch and peak amplification branch, thus lowering the efficiency and linear parameters of the whole power amplifier. Some manufacturers solve the above problem by adding a microwave adjustable capacitor in the input of the last-stage power amplifier, which not only increases the costs of the power amplifier but also greatly affects the manufacturability of large batch manufacture. The requirement by the operators on the communication system is lower power consumption and higher efficiency. Therefore, we have to seek for a method for further reducing power consumption and improving efficiency.